Oriented polyester film, particularly biaxially oriented film composed of polyethylene terephthalate (PET), is widely used as a base for drafting film, photographic film and reprographic film, as well as for packaging and labelling applications.
Because PET film is hydrophobic and is not readily receptive to coating in most applications where the film is to serve as a base or support for other coatings, it must be first coated on one or both sides with a primer coating which adheres to the film and is receptive as well to other coatings applied to it. For example, U.S. Pat. Nos. 2,627,088 and 2,698,240 teach a primer coating for PET film comprising a terpolymer composition of vinylidene chloride, acrylic ester and itaconic acid. This primer layer is said to have excellent adhesion to the polyester surface and to water or alcohol based photographic gelatin layers subsequently coated thereon.
Another known PET film primer includes copolymers of a vinylhalogenoester, such as vinylchloroacetate which may be copolymerized with numerous different monomers such as acrylic and methacrylic acids, esters and amides, olefins and vinyl alcohol as disclosed in U.S. Pat. No. 3,674,531. Such copolymers may also be crosslinked by the inclusion of melamine or urea formaldehyde resins in the composition. The primed PET film is stated to exhibit enhanced adhesion to a variety of coatings applied thereto, including reprographic coatings.
While some of these and other polyester film primer layers are effective in enhancing the adhesive qualities of PET film, it is important for the film manufacturer that scrap film made during production must be recyclable through the film-forming process. Scrap film is normally comminuted, melted, extruded into the form of pellets, mixed with fresh virgin polyester, re-melted and re-fed to the film-forming extruder. Temperatures of about 270.degree. C. to 310.degree. C. may be encountered during such processing of PET reclaim film. Many of the primer compositions discussed above are not stable at such temperatures and tend to impart an undesirable yellow or black discoloration to finished oriented PET film containing significant amounts of such primed reclaim film, particularly after repeated passes through the extruder. Such is the case with the vinylidene chloride-containing polymers used as PET primer layers and disclosed in U.S. Pat. Nos. 2,627,088 and 2,698,240. It is also the case with primer layers based on copolymers containing vinyl chloroacetate as disclosed in U.S. Pat. No. 3,674,538. It has been found that discoloration and degradation of these primer layers during the reclaim process is most likely attributable to the evolution of chlorine gas or hydrogen chloride in the case of chlorine-containing primer layers.
Another known primer is the thermoset, acrylic or methacrylic coatings taught in U.S. Pat. No. 3,819,773, which can be applied to the PET film in the plant from aqueous medium. Such a primer layer enhances the adhesion of organic solvent based reprographic and drafting layers applied thereto. This patent also discloses that film primed with the thermoset acrylic coatings described therein may be reclaimed in the film forming extruder by mixing it with 50% by weight or more of virgin polyester and refeeding the mixture to the film-forming extruder. Patentee indicates that problems of discoloration or degradation caused by certain prior art primer coatings are reduced. While this is true in comparison with the chlorine-containing primers discussed above, the acrylic primer coatings of this patent crosslinked using the resinous crosslinking agents disclosed therein still are found to give rise to an undesirable yellowing of finished film containing such primed film as reclaim, particularly when compared to film based solely on virgin polymer.